The invention relates to a drying apparatus for drying a fibre web, comprising two endless bands impermeable to air and having a good thermal conductivity, said bands being arranged to move in parallel with each other over a distance so that they form therebetween a drying zone where one band is heated by pressurized steam and the other band is cooled by pressurized water or some other efficient heat transfer medium, whereby the fibre web and at least one felt or wire are passed between the bands through the drying zone formed by the bands so that the fibre web is in contact with the heated band while the felt or wire is positioned between the fibre web and the cooled band.
It is known from e.g. Finnish Patents 66041, 76192 and 76856 to dry a fibre web between two parallel metal bands moving in the same direction so that the fibre web is in contact with a heated metal band and a felt is arranged between the fibre web and the other i.e. cooled metal band such that steam separating from the fibre web as a result of heating will condense in the felt under the influence of the cold metal band. The operation is based on two endless metal bands arranged to move around turning rolls and a pressure chamber formed against a surface remaining inside each one of the two loops formed by the metal bands. One pressure chamber contains hot steam while the other contains water so that pressure created presses the hot and the cold band and thus the fibre web and the felt positioned therebetween against each other. The bands, which are positioned between the pressure chambers, form one side in the pressure chambers by means of seals so that steam and water are able to act directly on the bands. The operation of the apparatus as such is fully known and has been described e.g. in the above-mentioned patents, which are incorporated herein by reference.
The problem with the prior art solutions is that especially the steam chamber is difficult to seal, as a result of which steam escapes from the chamber between its edge and the heated band on the side of the chamber, thus causing noise problems and other problems. What is even more important, it causes great temperature differences at the band edge, as steam condensing to a low pressure cools the band edges with resultant thermal stresses and bending or cracking of the edges, which is disadvantageous to the operation of the apparatus and may cause serious damage.
In an attempt to avoid such a problem, various sealing arrangements and other solutions have been used, which, however, often complicate the structure and make it difficult to realize.